Modern communications systems employ a wide spectrum of modulation techniques, such as CDMA (Code Division Multiple Access), WCDMA (Wideband CDMA), OFDM (Orthogonal Frequency Division Multiplexing), and the like. In order to avoid intermodulation production and spectral regrowth, it is essential that RF power amplifier circuits used in such systems be highly linear. However, modulated signals used in such modulation techniques have complicated signal statistics and high PAR (Peak-to-Average power Ratio). The signal statistics inherently affect the nonlinear behavior of high power amplifiers, and circuit configurations for linearization of the high power amplifiers should be adapted to the signal statistics including PAR.
Therefore, to achieve a required performance efficiently, an appropriate linearization strategy for each modulation scheme should be applied. Generally, feed-forward and various kinds of predistortion circuits have been widely used to linearize high power amplifiers.
In a predistortion scheme, a controlled nonlinear distortion is applied to an amplifier input signal. A predistortion circuitry is designed to give nonlinear amplitude and phase characteristics complementary to a distortion generated by the amplifier itself, so that ideally, the distortion is canceled out in the amplifier output over the entire signal bandwidth. The predistortion scheme however suffers from some drawbacks of poor linearity and amplification efficiency.
The feed-forward (FF) compensation scheme has been generally considered to be a best linearization scheme and thus an FF amplification apparatus has been most widely used to obtain better linearity in a high power amplifier.
FIG. 1 shows a block diagram of a conventional FF amplification apparatus 100.
An input signal of the FF amplification apparatus 100 is divided into a main path signal and a subsidiary path signal at a power splitter 10. The main path signal is provided to a main amplifier 14 via a first vector modulator 12, and the subsidiary path signal is provided to a combiner 20 via a first delay line 18. The main path signal is then amplified by the main amplifier 14, and transferred to a first directional coupler 16, wherein the amplified main path signal includes therein distortion components generated in the amplification process. The amplified main path signal is separated into a primary signal and an auxiliary signal at the first directional coupler 16.
The primary signal is delayed and forwarded to a second directional coupler 28 via a second delay line 26, whereas the auxiliary signal is sent to the combiner 20. The auxiliary signal is combined with the delayed subsidiary path signal at the combiner 20 to extract therefrom an error signal corresponding to distortion components generated in the main amplifier 14.
The error signal is provided to an error amplifier 24 via a second vector modulator 22, and then transferred to the second directional coupler 28. The delayed primary signal is coupled with the amplified error signal at the second directional coupler 28, and thus amplified distortion free signal is outputted therefrom.
However, the FF amplification apparatus also has many drawbacks, such as complexity, efficiency, size, and so on, which result in cost problems. Also, linearity specifications of the 3rd generation wireless systems especially ACLR (Adjacent Channel Leakage Ratio) and out-of-band spectrum emission, become more stringent than those of the 1st or 2nd generation systems. Thus, a linearization technique, which provides a better performance than FF and predistortion, is highly desired.